Not Applicable
Not Applicable
Not Applicable
1. Field of Invention
The present invention relates to a mechanical means for defrosting plates used in cold storage.
2. Description of Prior Art
Cold storage systems used in the transportation of refrigerated or frozen goods are made up of plates containing a phase change material (eutectic plates) and a system which produces cold. When the truck is idle, the cold is produced and stored in eutectic plates located in the cargo space of the truck. When the vehicle is on the road, cold is no longer produced and the cold stored in the eutectic plates is used.
In the text of this patent, the plates are said to be charged when the solution inside the plates is solid (low enthalpy) and discharged when the solution is in liquid form (high enthalpy). The plates are discharged by transferring the heat from the surrounding environment to the solution, whereby it melts. The plates are charged through a refrigeration system that withdraws the heat from the solution, whereby it becomes solid.
Typically, the plates are assembled in parallel and side by side and forced air circulates between them. The air circulating between the plates is the air produced in the truck""s cargo space. This air is warmer than the surface of the plates in order to promote the transfer of heat and it is likely to have a dew point temperature that is higher than the temperature of the surface of the plates, which inevitably results in the formation of frost on the surface of the plates. Frost impairs the performance of the apparatus because it reduces the transfer of the heat of the air to the plates and partially or totally obstructs the passage of air.
In general, the defrosting of cold storage plates (eutectic plates) used in refrigerated trucks is performed through the following procedures:
Heating Elements: Electric heating elements are glued to the sides of the plates and melt the frost. The power required to rapidly defrost the plates is significant. In U.S. Pat. No. 5,172,567, heating elements are directly built into the plates.
Hot Water or Sprayed Steam: Hot water or steam is sprayed onto the frost to melt it. Pipes are permanently installed on the apparatus. The hot water and steam are provided through an external source. The water and melted frost fall into a container at the base of the apparatus.
Hot Gas: The surface of the plates is heated by using the refrigeration system""s condensing unit. The refrigerant from the condenser is channeled through tubes which are in contact with the surface of the plates. When the refrigerant circulates through the tubes, at a high pressure and temperature, it heats the surface of the plates and melts the frost. This method is described in U.S. Pats. Nos. 4,043,144 and 4,110,997. These three methods encompass significant disadvantages, such as:
The defrosting process must be performed at the loading dock because a significant amount of energy is required either to start up the refrigerating unit, to produce the hot water and sprayed steam or to provide the electrical power for the heating elements.
Electric defrosting requires a substantial amount of power and soon becomes costly to operate.
Hot water and sprayed steam can, in certain cases, damage the shipment in the refrigerated truck. The truck must therefore be unloaded before starting the defrosting process. The water or the defrosting solution must be properly treated, destroyed or recycled.
In the three methods described above, part of the heat used to melt the frost seeps into the cargo space which results in an undesirable increase in the temperature of the refrigerated trailer.
By applying heat to the surface of the plates, these three methods promote the heating of the cold storage solution inside the plates and reduces the level of residual charge. Consequently, the time required to completely recharge the plates is much longer. This results in a significant waste of energy and time before the truck can be loaded.
The method proposed in this patent does not use any heat source to melt the frost. The frost is simply removed from the surface and falls into a container at the base of the plates. This container is emptied periodically. The frost is removed from the surface of the plates by scrapers that sweep the plates horizontally. The method described in this patent allows for defrosting to be performed while the plates are being discharged which improves the thermal performance of the cold storage system.
This defrosting system is used at temperatures which do not exceed the freezing point of water. As long as the surfaces to be defrosted remain below the freezing point, the density of the surface frost which is formed is low and the frost is friable. If the temperature of the surface or the accumulated frost becomes higher than the freezing point and subsequently becomes lower, ice will form and the scrapers will not perform properly. In such cases, the entire system should be brought to a temperature which is higher than the melting point of water in order to melt the ice and accumulated frost. Under normal circumstances, eutectic plates are charged during the night and discharged during the day. However, the amount of residual cold at the end of the discharging process is sufficient to maintain the temperature of the surface of the plates at a lower level than the temperature of the frost""s melting point. It is therefore possible to effect several charging/discharging cycles without having to bring the surface of the plates to a higher temperature than that of the temperature of the frost""s melting point. Conserving the residual cold at the end of the charging process results in a saving of time and energy for subsequent charging.